Throughout this disclosure, notations and examples based on the Universal Mobile Telecommunication Standard, Long Term Evolution (UMTS-LTE, also termed Evolved UMTS Terrestrial Radio Access Network—E-UTRAN—and advocated by the Third Generation Partnership Program—3GPP) will be used. It should be noted, however, that this is not meant as being limiting. Contrarily, embodiments and application scenarios may be equally applicable in relation to other known or future communication standards. DL denotes downlink and UL denotes uplink.
Carrier Aggregation
Carrier aggregation was introduced in Release 10 of the E-UTRAN standard for E-UTRAN to be able to meet the 1000 Mbit/s requirement of 4G. Another purpose of carrier aggregation is to allow operators with small (e.g. less than 20 MHz) and scattered spectrum allocations to provide a good user experience by aggregating the scattered allocations into e.g. 10 MHz, 20 MHz, or more.
In a typical carrier aggregation example, the User Equipment (UE) is connected to a serving cell, which is termed the Primary Cell (PCell) using a carrier referred to as the Primary Component Carrier (PCC). Mobility is catered for on the Primary Component Carrier. If the UE is using services that require high throughput, the network may activate one or more additional serving cells, each termed Secondary Cell (SCell), using respective carrier(s) referred to as Secondary Component Carrier(s) (SCC). The Secondary Cell activation may happen before or after the SCell has been detected by the UE. For example, SCell activation before SCell detection may be based on knowledge by the network regarding the position (location) of the UE. For example, the network may be aware that the UE is located in a coverage area of a potential SCell. Examples of such an approach to SCell activation are disclosed in WO/2012/154112.
Two types of carrier aggregation scenarios are considered for Release 10:                Intra-band contiguous aggregation.        Inter-band aggregation.        
In Release 11, one additional type is considered:                Intra-band non-contiguous aggregation.        
In intra-band contiguous aggregation, the PCell and SCell(s) are contiguous in frequency. It is required by the standard that (for intra-band contiguous aggregation) the time (or equivalently, timing) difference between PCell and SCell is at most ±130 ns (3GPP TS 36.104 rev 11.4.0, subclause 6.5.3). It is further assumed in the standard that, for this particular scenario (intra-band contiguous aggregation), one can use a single fast Fourier transform (FFT) to simultaneously demodulate the signals received from PCell and SCell. Thus, in practice it is typically required that the PCell and SCell are co-located, i.e., transmitted from the same (geographical) site, since otherwise the difference in propagation delays between PCell and SCell would make it impossible to use a single FFT.
For intra-band non-contiguous aggregation the timing difference is allowed to be at most ±260 ns, and no assumption is made regarding the cells being co-located or regarding that it should be possible to use a single FFT.
For inter-band carrier aggregation the timing difference between the PCell and SCell is allowed to be at most ±1.3 μs. For this scenario it is further assumed that the cells may be non-co-located and that the UE should be able to cope with a propagation delay difference between PCell and SCell of up to ±30 μs, resulting in a maximum delay spread of ±31.3 μs (3GPP TS 36.300 rev 11.5.0 Annex J).
Examples of foreseen deployment scenarios are shown in FIG. 1 (see also 3GPP TS 36.300 rev 11.5.0 Annex J). Solid lines illustrate PCell on a carrier frequency F1 and dashed lines illustrate SCell on a carrier frequency F2.
Part (a) of FIG. 1 illustrates a co-located overlaid intra-band scenario, a scenario with fully overlapping coverage of PCell and SCell. Three base station sites 101a, 102a, 103a are illustrated, each providing three PCells (as illustrated by 121:1a, 121:2a, 121:3a for the site 101a) and three SCells (as illustrated by 131:1a, 131:2a, 131:3a for the site 101a). Since the different carrier frequencies of PCell and SCell are in the same frequency band, the path losses experienced in PCell and SCell respectively are comparable and, hence, the coverage area of PCell and SCell are similar.
Part (b) of FIG. 1 illustrates a co-located overlaid inter-band scenario. Three base station sites 101b, 102b, 103b are illustrated, each providing three PCells (as illustrated by 121:1b, 121:2b, 121:3b for the site 101b) and three SCells (as illustrated by 131:1b, 131:2b, 131:3b for the site 101b). Since the different carrier frequencies of PCell and SCell are not in the same frequency band, the difference between path losses experienced in PCell and SCell respectively is large and, hence, the coverage area of PCell and SCell are different.
Part (c) of FIG. 1 illustrates a co-located, partially overlaid, inter-band scenario. Three base station sites 101c, 102c, 103c are illustrated, each providing three PCells (as illustrated by 121:1c, 121:2c, 121:3c for the site 101c) and three SCells (as illustrated by 131:1c, 131:2c, 131:3c for the site 101c). The coverage area of PCell and SCell are different.
Part (d) of FIG. 1 illustrates a non-co-located inter-band scenario. Three base station sites 101d, 102d, 103d are illustrated, each providing three PCells (as illustrated by 121:1d, 121:2d, 121:3d for the site 101d). Further, there are remote radio heads (e.g. 111:1d, 111:2d) each providing a PCell (as illustrated by 131:1d for 111:1d and by 131:2d for 111:2d) to provide improved throughput at hotspots. The coverage area of PCell and SCell are different.
Part (e) of FIG. 1 illustrates a co-located overlaid inter-band scenario with repeaters. Similarly to part (b), three base station sites 101e, 102e, 103e are illustrated, each providing three PCells (as illustrated by 121:1c, 121:2c, 121:3c for the site 101c) and three SCells (as illustrated by 131:1c, 131:2c, 131:3c for the site 101c), wherein the coverage area of PCell and SCell are different. Further, there are repeaters (e.g. 111:2d) each providing a PCell (as illustrated by 141:2d for 111:2d) to provide improved throughput at hotspots.
For co-located intra-band scenario with fully overlapping coverage area of PCell and SCell (see FIG. 1, part (a)), the eNB (enhances NodeB) can configure and activate the SCell when needed, based on reported measurements for PCell alone.
More generally, the timing of the SCell is priorly known if the UE has measured and reported that cell recently (the exact time frame, i.e. the definition of recently, is currently (2013) under discussion in standardization work), either as an inter-frequency neighboring cell or as a cell on a configured secondary component carrier (F2). The timing of the SCell is also considered priorly known, regardless of whether or not it has been reported recently, for intra-band contiguous carrier aggregation, i.e., where the spectrums of PCell and SCell are contiguous (“back-to-back”). When the UE gets an activation command for the SCell under any of those conditions, the UE may be able to start reception from the SCell without first performing fine-tuning of the timing.
If the cell corresponding to SCell has not been reported recently and is either on another band (inter-band aggregation scenario) or intra-band non-adjacent, the timing of the SCell is not known to the UE. However, as mentioned above it is known that it shall fall within ±31.3 μs (almost half an OFDM—orthogonal frequency division multiplex—symbol) relative to the PCell. In this case, the timing of the SCell will have to be tuned before the UE can start reception from the SCell.
Various techniques for SCell synchronization are known. For example, WO2013/025547A2 discloses some situations where the PCell timing may be used as initial time synchronization for SCell.
Cell Detection and Measurements
Connected mode mobility within E-UTRA is supported by measurements that are carried out and reported to the network by the UE. The network uses the measurement reports when deciding whether or not to hand over the UE to another eNB. The network may typically also use the measurement reports for other purposes such as e.g. network optimization and cell planning.
The measurements carried out by the UE typically include detection of neighboring cells (e.g. cell search) and estimation of signal strength (e.g. Reference Signal Received Power—RSRP—and/or Reference Signal Received Quality—RSRQ). The requirements in 3GPP TS 36.133 rev 11.4.0 subclause 8 stipulates that the UE shall be able to detect intra-frequency neighboring cells that are as weak as Ês/Iot=−6 dB (frequency-domain Signal-to-Interference-and-Noise Ratio—SINR) and report them to the network within a given time. For carrier aggregation, both PCC and SCC are considered as intra-frequency carriers and the corresponding cells are considered as intra-frequency neighboring cells, which should be detected and reported correspondingly.
Cell search in E-UTRA comprises acquisition of: frequency and symbol synchronization, frame synchronization, and physical cell identity. It is facilitated by two signals that are transmitted on a 5 ms basis (repeated every 5 ms) in each cell: the primary synchronization signal (P-SSIG or PSS) and the secondary synchronization signal (S-SSIG or SSS). These signals are described in 3GPP TS 36.211 rev 11.2.0 subclause 6.11, and illustrated in FIG. 2 for an LTE FDD (frequency division duplex) radio frame. Further details around the central 72 subcarriers are highlighted in FIG. 3 for LTE FDD (frequency division duplex) and in FIG. 4 for LTE TDD (time division duplex). The illustrations in FIGS. 2, 3, and 4 are schematic and hence not meant to be in exact scale or correct with regard to mutual sizes of items.
FIG. 2 illustrates an example time-frequency layout of an LTE FDD radio frame. Time is illustrated in the horizontal axis (subdivided into subframes 0, 1, 2, . . . , 9 as illustrated by 150, 151, 152, 153) and frequency is illustrated on the vertical axis. The black boxes illustrate Cell-specific Reference Signals (CRS) that are always present and the white boxes illustrate Cell-specific Reference Signals (CRS) that are present sometimes. The central 72 subcarriers are illustrated by 154, and a part of thereof is enhanced at 158 for clarity. The transmission of SSS is shown at 155, the transmission of PSS is shown at 156, and the transmission of the Physical Broadcast CHannel (PBCH) is shown at 157.
FIG. 3 illustrates example synchronization signals and reference symbols transmitted in an FDD cell (compare with FIG. 2). Only the central 72 sub-carriers are shown. Time is illustrated in the horizontal axis (subdivided into subframes 0, 1, 2, . . . , 9 as illustrated by 160, 161, 162, 163) and frequency is illustrated on the vertical axis (subdivided into resource blocks 0, . . . , 5 as illustrated by 164, 165). The black boxes illustrate CRS for down-link transmission (DLTX0, from antenna port 0 of the base station) that are always present and the white boxes illustrate CRS for DLTX0 that are present sometimes. The transmission of SSS and PSS (transmitted on the Secondary Synchronization CHannel—S-SCH and Primary Synchronization CHannel—P-SCH, respectively) are shown in the same positions as in FIG. 2. Some sub-frames (illustrated by 166 and 167) may be used for e.g. MBSFN (Multicast Broadcast Single Frequency Network). Therefore, they might not contain cell-specific reference symbols.
FIG. 4 illustrates example synchronization signals and reference symbols transmitted in a TDD cell. Only the central 72 sub-carriers are shown. Time is illustrated in the horizontal axis (subdivided into subframes 0, 1, 2, . . . , 9 as illustrated by 170, 171, 172, 173) and frequency is illustrated on the vertical axis (subdivided into resource blocks 0, . . . , 5 as illustrated by 174, 175). The black boxes illustrate CRS for DLTX0 that are always present and the white boxes illustrate CRS for DLTX0 that are present sometimes. The transmission of SSS and PSS (transmitted on the Secondary Synchronization CHannel—S-SCH 178 and Primary Synchronization CHannel—P-SCH 179, respectively) are also shown. It may be noted that the positions of the synchronization channels differ slightly from the FDD case. Some sub-frames (illustrated by 176 and 177) may be used for e.g. guard period (GP) purposes, uplink (UL) or MBSFN. Therefore, they might not contain cell-specific reference symbols.
As described in the standard, the P-SSIG exists in three versions (one for each out of three cell-within-group identities), based on Zadoff-Chu sequences that are mapped onto the central 62 resource elements (REs) (only 62 out of the 72 central REs may be used and the 5 REs closest to each edge set to zero according to a typical approach). There are 168 cell groups in total, and information regarding which cell group a cell belongs to is carried by the S-SSIG, which is based on m-sequences. The cell group and the P-SSIG version together yield the physical cell identity, of which there are 3×168=504. S-SSIG also carries information regarding whether the particular instance of S-SSIG is transmitted in subframe 0 or subframe 5, which is used for acquiring frame timing (frame synchronization). Furthermore, the S-SSIG is scrambled with the cell-within-group identity of the particular cell. Hence, there are 2×3×168=2×504 versions in total, two for each of the 504 physical cell identities. Similarly to P-SSIG, S-SSIG is mapped onto the central 62 REs.
Detection of a cell may, as is well-known in the art, be based on matched filtering of received samples (based on the three P-SSIG versions) over at least 5 ms. Correlation peaks in the matched filter output typically reveal synchronization signals from one or more cells. This procedure is referred to as symbol synchronization. Having established symbol synchronization and identified the cell-within-group identity, 5-SSIG detection is typically carried out yielding frame timing (frame synchronization), physical cell identity (via determination of the cell group identity), and cyclic prefix (CP) configuration.
Requirements of SCell Activation
A typical example requirement on the maximum acceptable delay (latency) for SCell activation (from reception of the activation command until valid channel state information (CSI) is transmitted to the network) may comprise that, for SINR>−3 dB, activation shall be completed within                24 ms if the cell is known (typically defined as e.g. RSRP measurements having been reported to the network within the last period of a length equal to the minimum of 5 DRX cycles and 5 SCell measurement cycles), and        34 ms if the cell is unknown, often referred to as blind activation (typically defined as e.g. RSRP measurements not having been reported within the period above).        
According to the standard, the UE shall start transmitting CSI already 8 ms after having received the SCell activation command. However, before synchronization to the SCell has been achieved, the CSI shall indicate out-of-range (Channel Quality Indicator, CQI=0).
The example requirement shall typically be met for a worst case scenario regarding the available number of unicast subframes. For LTE FDD the worst case scenario is when there are 2 unicast subframes per 5 ms. For LTE TDD the worst case scenario is when there is only one unicast subframe and one special subframe per 5 ms.
It may be challenging to meet a typical blind SCell activation requirement if typical, known cell detection approaches (e.g. the ones disclosed in WO2013/025547A2) are used. Therefore, there is a need for methods and arrangements that can achieve CSI reporting as required. For example, there is a need for methods and arrangements that can meet the latency requirements of CSI reporting during SCell activation in carrier aggregation scenarios